Solifenacin succinate is a commercially marketed pharmaceutically active substance indicated for the treatment of overactive bladder with symptoms of urinary incontinence, urgency and high urinary frequency. Solifenacin succinate is the international common denomination for butanedioic acid compounded with (1S)-(3R)-1-azabicyclo[2.2.2]oct-3-yl-3,4-dihydro-1-phenyl-2(1H)— isoquinolinecarboxylate (1:1), having an empirical formula of C23H26N2O2.C4H6O4 and the structure represented in formula I given below.

Solifenacin and its pharmaceutically acceptable salts are reported in U.S. Pat. No. 6,017,927 (the '927 patent).
The following Scheme 1 shows the synthetic routes disclosed in the '927 patent for the preparation of (1RS,3′RS)-solifenacin and (1S,3′RS)-solifenacin:

However, the processes described in the '927 patent for the preparation of solifenacin are not very efficient or suitable for industrial scale-up because they include laborious and costly work-ups. Further, since the ethyl carboxylate is used as the starting material in the process of Route A, ethanol is by-produced, and the by-produced ethanol launches a nucleophilic attack against solifenacin in the presence of a base. Thus, it is necessary to carry out the reaction while removing the ethanol from the reaction system, for example by the toluene azeotrope or the like, so that the control of the reaction is troublesome.
The following Scheme 2 shows the synthetic route disclosed in WO2005075474 for the preparation of solifenacin and solifenacin succinate:

However, the process described in this reference does not overcome the drawbacks associated with the state of the art processes.
Processes described in the prior art for the preparation of solifenacin and solifenacin succinate are not very efficient or suitable for industrial scale-up because they include expensive operations such as refluxing a mixture at high temperature for hours and laborious and costly work-ups including solvent exchanges and purification steps. So, there is a need for an improved and simplified process for the preparation of solifenacin and/or one of its salts that avoids the drawbacks of current state of the art processes.